1. Field of the Invention
The present invention relates to a substrate for a light-emitting device and an optical device using the same.
2. Description of the Related Art
An organic electroluminescence (EL) device is a spontaneous emitting display device employing a principle that fluorescent material emits light in response to recombination energy of holes injected from an anode and an electrons injected from a cathode when an electric field is applied. After a layered low-voltage driven organic EL device is reported, studies on the organic EL device using an organic material as a constituent material have been briskly performed.
Tang et al. teaches that an emitting layer is formed of a tris(8-quinolinol)aluminum, and a hole transporting layer is formed of a tri-phenyl thiamine derivative. The layered structure has advantages, for example, in that efficiency of injecting the holes into the emitting layer is increased, that efficiency of creating excitons by means of the recombination is increased by blocking electrons injected from the cathode, and that the created excitons are confined in the emitting layer.
In this manner, as a structure of the organic EL device, there has been widely known a two-layered structure composed of a hole transporting (injecting) layer and an electron transportable emitting layer, or a three-layered structure composed of a hole transporting (injecting) layer, an emitting layer and an electron transporting (injecting) layer. In order to enhance recombination efficiency of injected holes and electrons in the device having the layered structure, structure of the device and method of fabricating the device has been studied.
However, in the organic EL device, the upper limit of a light-emitting probability exists because of restriction on a singlet formation probability rather than dependency on the spin statistics during recombination of carriers. The upper limit is known to be about 25%.
Further, in the surface light-emitting device having a wave front of a spherical wave form, such as the organic EL device, in which the emitting layer is at least interposed between the anode and the cathode, because a light-emitting member has a refractive index higher than that of a substrate or air, light having an exit angle larger than the critical angle causes the total reflection at a substrate/air interface, so that the light cannot be taken out. It is known that, when the refractive index of the light-emitting member is 1.6, only about 20% of the emitted light can be effectively utilized.
For this reason, the upper limit of the total energy conversion efficiency including the singlet formation probability is forced to be about 5% in total. In the organic EL device in which the light-emitting is strictly restricted, the low efficiency of the light emitting incurs decrease of the energy conversion efficiency considered to be fatal.
Conventionally, several proposals have been provided as an approach for elevating the light emitting efficiency.
For example, Japanese Patent Publication No. S63(1988)-314795 discloses a method of forming a lens on a substrate. Here, the substrate makes use of one having capability of collecting light such as a cell fork or a convex lens.
Japanese Patent Publication No. H01(1989)-200394 discloses a method of forming a reflection surface as a means for enhancing the light emitting efficiency. Here, it is characterized in that a mirror reflecting light is provided on one surface of an emitting layer and takes a mortar form. Therefore, a loss of light leaking out around the emitting layer is improved.
Japanese Patent Publication No. H13(2001)-202827 proposes a method of disposing a layer having a low refractive index between a substrate and an electrode layer. The disclosed technology includes a transparent dielectric layer (electrode layer) in contact with at least one surface of the low refractive index layer, and realizes enhancement of an emitting rate of propagating the light outside because the light passing through the low refractive index layer is allowed to enhance the emitting rate to the air, enhancement of an emitting rate of propagating the light outside because a refractive index of the low refractive index layer is from 1.003 to 1.300 and thus the light passing through the low refractive index layer is allowed to enhance the emitting rate to the air, and a very low refractive index approaching 1 (one) because a silica aerogel is used for the low refractive index layer.
Japanese Patent Publication No. H15(2003)-31374 discloses a light-emitting device having an anti-reflection layer in which high refractive index layer, low refractive index layer and high refractive index layer are deposited between a substrate and an emitting layer in that order. FIG. 1 of the document shows a structure having an optical multilayer element 2 where the high refractive index layer H1, the low refractive index layer L1, and the high refractive index layer as an anode layer 2A are deposited in that order between the substrate 1 formed of glass and an organic EL layer 3. And, the paragraph [0025] discloses that the optical multilayer element 2 has an anti-reflection function and thus efficiency of extracting the light from the substrate 1 is improved.
When a so-called anti-reflection layer is composed of a single layer and satisfies nd=λ/4 (where d is the thickness of the physical layer and λ is the used wavelength), reflection light is eliminated by interference of the light. The technology described in the document is a kind of this anti-reflection layer and has a multi-layered structure. In the structure described in the embodiment, the high refractive index layer H1, the low refractive index layer L1 and the anode layer 2A have 14.2 nm, 41.5 nm and 139.8 nm in thickness, respectively (paragraph [0015]). The thickness of each layer is set to be less than a half wavelength with respect to the light-emitting wavelength of 400-700 nm. The multilayer disclosed in the document is a kind of so-called anti-reflection layer.
However, the conventional technologies leave additional room for improvement in the following respects.
Both the method of forming a lens on a substrate and the method of forming a reflection surface, which are described in Japanese Patent Publication Nos. S63-314795 and H01-200394 respectively, are effective for a large-sized device having a large emitting area, but has a difficulty in forming a lens having capability of collecting light, a lateral reflection surface, etc. in a small-sized device having a small pixel area, such as a dot matrix display. Particularly, in the organic EL device, because the thickness of the emitting layer is less than several microns, it is difficult not only to form the reflection mirror, which has a tapered surface on the lateral surface of the device, even by use of the current technique, but also to avoid sharp increase of cost.
The method of disposing a layer having a low refractive index between a substrate and an electrode layer, which is described in Japanese Patent Publication No. 2001-20827, is effective in that the light emitting efficiency is improved by collection of the light within the critical angle. However, since the light is reflected at the interface between the anode and the low refractive index layer, improvement of the light emitting efficiency is still considered to be insufficient.
Further, in the case of using the polished silica aerogel layer in order to obtain the very low refractive index layer, a mechanical strength of the layer is very weak. Further, a short of the electrode caused by surface unevenness of the polished layer is generated, so that portions where the light is not emitted (dark spots) are generated. Light extracting technique effective for the organic EL device does not still reach a satisfactory level.
The method of providing an anti-reflection layer, which is described in Japanese Patent Publication No. 2003-31374, has a limitation to improvement of the light emitting efficiency in view of its function (which will be described in the present embodiments).
Further, as known, because the anti-reflection layer has high dependency on the wavelength, the structure has the light emitting efficiency changed to a great extent by the emission wavelength. For this reason, when the technique of the document is employed to the light-emitting device for a white color, a quantity of light emitted outside the substrate is greatly changed by the wavelength, so that deterioration of white balance becomes problematic.
Moreover, because the anti-reflection layer removes reflection of the light using the interference of the light, it is necessary to form the thickness or the refractive index of the layer so as to satisfy a predetermined condition. Therefore, in the case that the thickness of the layer is slightly changed by fabrication factors, the reflectivity of the anti-reflection layer is changed. As a result, the light-emitting device is vulnerable to an error in its performance.